June 28, 2010
Even though the recent focus has been on Tropical Storm Alex and its development from a tropical wave, the western hemisphere hurricane season, up to this point has been an eastern Pacific show, starting with Agatha taking a rarely seen path into Central America and causing devastating flooding in Guatemala. The formation of the second tropical depression of the season on June 16 marked the beginning of a burst activity coinciding with a favorable intra-season oscillation. Over the next seven days, three more tropical cyclones would form, Tropical Storm Blas and the Hurricanes Celia and Darby.
Blas is long gone. The once-mighty Celia is literally a shell of her former self:
And while for the past few days, we’ve been able to see Darby off the coast of Mexico in the western Atlantic image, it has been subsumed by the mighty reach of our Atlantic storm.
I’ll start with the recent past of the track forecast as guided by the computer models. This morning’s models featured a narrowed spread among forecasts as the right (northern) outliers continued to bring their forecasts to the left (south), while others began to cluster. The spread of the most reliable of the models ranged from the Laguna Madre in northern Mexico to Corpus Christi, TX.
Graphic courtesy of Jonathan Vigh’s model guidance page
After being to the south of the model guidance for most of Alex’ life, the center of the forecast track lay a bit to the north of several models, and this was mentioned in the 5 PM discussion. That would suggest a move further to the left/south for the 10 PM CDT advisory, but recent events put that in doubt.
One being the 18Z specialized dynamic GFDL and HWRF models. The HWRF remained steadfast in its forecst, bringing the storm to Corpus Christi. The GFDL, whose forecast had been one of those to the left of the National Hurricane Center’s shifted to the north, bringing Alex ashore just south of the U.S. border with Mexico.
The other “event” was Alex’ motion today, or the lack thereof. While expected to have a relatively distinct movement to the northwest, the fixes from the morning Hurricane Hunter flight showed the center to be meandering, with only a few nautical miles of movements between fixes that were spaced a couple of hours apart. After the mission ended early this afternoon, it appeared that the lack of a definitive steering current, combined with the heavy thunderstorm activity in the northeast quadrant of the storm was causing the center of Alex to be pulled to the east/northeast. Until this evening’s mission entered the storm and obtained a center fix, it wasn’t clear whether this was an optical illusion caused by shear or the apparent movement was indeed real. The last fix of the afternoon flight was 20.3N 91.7 W. The first fix of this evening’s mission was 20.7N 91.6W. Net movement between fixes: 25 nautical miles to the north-northeast. Not a big movement, considering that there was over seven hours between fixes, but for a storm that was supposed to be moving northwest, that does introduce some immediate error in the models’ forecasts. While no model grabbed this, (which would have been unlikely due to it being partially small scale dynamics driven), it looks like the HWRF, by virtue of having the least amount of westerly movement in the early part of its forecast, is best on track early on in the forecast period. This ought to keep the 11 PM advisory forecast track relatively fixed left/right wise.
As I mentioned yesterday, NOAA’s Gulfstream jet flew a mission this evening to collect upper air data over the Gulf of Mexico. That data will be fed to the 00Z run of the forecast models this evening, which guides the NHC forecast released early tomorrow morning. We may see some movement in the models forecast because of the data, but hopefully not an opening up of the spread.
As for the intensity forecast… this morning’s mission showed a steady state storm with a pressure of 990 millibars and winds of 50 knots/60 mph. Satellite imagery this afternoon did not show any appreciable changes, as Alex appeared to be fighting a bit of shear and dry air and maybe was being hampered by being in shallow waters. However, as you can tell by comparing the western Atlantic image above from mid-afternoon, with the focused shot of Alex from early this evening, Alex came to life as the daily minimum of thunderstorm activity passed. Recon’s center fix found pressure had dropped to 987 millibars, indicating a strengthening storm. The plane has yet to sample the northeast quadrant where the strongest winds are located. Needless to say, what it finds has an immediate effect on the short term forecast as the initial intensity will probably be higher.
SHIPS intensity guidance, which the NHC forecast has mirrored, dropped a little bit today and now indicates a peak intensity of around 80 knots prior to landfall (assuming the NHC’s forecast track). It shows Alex at 56 knots tomorrow morning and at hurricane strength by this time tomorrow night. Depending on what the recon mission finds on its northeast quadrant pass, it could be behind the ball already (it wouldn’t surprise me if the plane found 55 knot surface winds), especially if the apparent deepening pressure trend continues. If Alex really is intensifying right now, it would be a bit disconcerting because it hasn’t rid itself of negative factors yet. It’s still in shallow water (with a limited amount of heat potential) and it is facing a fair bit of shear from an unfavorable direction. As it moves away from land, it finds deeper water with greater heat potential. Also, the GFS forecast fed to SHIPS indicates that shear becomes lighter as the forecast period goes on and shifts to a more favorable direction (enhancing outflow). All of the above would suggest Alex capable of becoming a major hurricane.
Texas residents should be keen to take a look at the National Hurricane Center forecast in the morning (remembering to focus more on the cone of uncertainty rather than the center line!). The window of opportunity for preparation making is narrowing and time is becoming of the essence.
I intend to have a quick post or two to my Friendfeed as the recon mission continues and the 10 PM advisory comes out. Also, I will (yes, will), have at least a quick post here early tomorrow morning.